The present invention relates to a serial printer of the type having a carriage movable along a printing line and a character-bearing element rotatable on the carriage for selecting the character to be printed at each printing point along the printing line. The character-bearing element may always be stationary at the instant of printing.
A serial printer of this type is that disclosed in U.S. Pat. No. 4,101,006 wherein provision is made for increasing the printing speed by avoiding stopping the carriage at the printing point. The printing is therefore on-the-fly so far as the carriage is concerned, although the character-bearing element is stationary on the carriage. The expression on-the-fly is always used herein in relation to the carriage.
It is necessary that the time taken by the carriage to cover the space between one printing point and the next be sufficient to permit the character-bearing element to position itself at the fresh character; in other words, it is necessary that the time of movement of the carriage be greater than, or equal to, the time of selection of the character.
To this end, the carriage is decelerated from a predetermined speed to a lower speed and then accelerated to the same speed again, before reaching the printing point. It is therefore necessary to have a complex control device capable of generating a family of speed curves for the carriage. Each curve is predetermined as a function of the angle of rotation of the character-bearing element required to select the new character to be printed. A serial printer of this kind has rigid printing characteristics.
The main object of the present invention, therefore, is to achieve an increase in speed in a printer of the type indicated by means of a system which is simpler and has a greater flexibility of application.
The guiding principle is to move the carriage always at the maximum speed possible and to select printing on-the-fly only when the time of selection of the character is less than the time of movement of the carriage. When these conditions are not verified, arrest of the carriage is effected and printing is effected after selection is at an end.
In practice, it is only necessary to determine whether selection has been completed when the carriage reaches the printing point and if so to print on the fly. No consideration has to be given to the carriage speed or the character spacing. The result is that the system can also easily be used when it is desired to vary the space between one print position and the next or it is desired that this space be chosen as a function of the character to be printed. More particularly, the system described hereinafter effects both printing with one or more fixed spacings and spacing proportional to the width of the character to be printed.
When using a stepping motor, the carriage can be moved from one print position to the next by means of a variable number of steps of the motor.
Finally, the invention is easily applied and in particular can be employed both in printers of new design and in existing printers for increasing the printing speed thereof.